1. Field of the Invention
The invention relates generally to electrical solenoids that produce a linear, axial force, and, more specifically, to that class of electrical solenoids known as force motors which produce a relatively short displacement which is proportional to a driving current.
2. Description of the Prior Art
Solenoids are generally characterized by an actuation direction which does not change with regard to the direction of the energizing current. In other words, if a direct current supply has its polarity reversed, the solenoid still provides axial movement in the same direction.
Force motors are distinguished from solenoids in that they use a permanent magnet field to pre-bias the air gap of a solenoid such that movement of the armature of the force motor is dictated by the direction of current in the coil. Reversal of the polarity of current flow will reverse the direction of the force motor armature displacement.
Force motors are frequently used to drive a valve spool in a high performance aircraft where efficiencies of weight, size, cost and power consumption are of prime consideration. It is, therefore, advantageous to minimize losses associated with producing high magnetic forces and to minimize the size of the permanent magnets which normally have relative costs higher than the solenoid iron.
FIG. 1 in the present application illustrates a conventional force motor with a simplified construction for ease of explanation. A stator 10 includes mounting brackets 12 and an iron core which provides a path for flux travel. The armature 14 is mounted on and moves with output shaft 16. Included in the stator mount is permanent magnet 18 which generates a flux flow through the stator and the armature as indicated by the solid line arrows 20. This flux from magnet 18 travels in opposite directions across air gaps 22 and 24. Coils 26 and 28 are provided and are wound so as to provide flux flow paths indicated by dotted line arrows 30 which cross air gaps 22 and 24 in the same direction. Operation of the prior art force motor provides an output movement by shaft 16 when current in one direction is provided to coils 26 and 28 and movement of the output shaft in the opposite direction when the opposite current flow is provided to coils 26 and 28. This movement direction is caused by the fact that, as shown in FIG. 1, flux flow generated by the permanent magnet 18 (shown by solid line arrows 20) is in the same direction as coil generated flux flow (indicated by dotted line arrows 30) across air gap 22, but in an opposite direction across air gap 24. This causes a greater attraction at air gap 22 than would exist at air gap 24, and, thus, the armature is attracted towards the left-hand stator portion moving the output shaft to the left. Obviously, if the current flow in both coils 26 and 28 were reversed, the direction of the coil generated flux flow paths shown by dotted line arrows 30 would be reversed for both air gaps 22 and 24. It is noted that the permanent magnet 18 can be mounted in the stator assembly, as shown, or may be part of the armature.
If the coil generated flux flow were reverse (by winding the coil differently or merely reversing the polarity of the direct current supply) the flux flow would be cumulative across air gap 24 and differential across air gap 22, resulting in the armature movement to the right and consequent output shaft movement to the right. Air gaps 22 and 24 are designated working air gaps in which the flux passes through an air gap and, as a result, generates an attractive force between the stator and armature which is in the axial direction. The prior art force motors also have an additional air gap 32 which may be characterized as a non-working air gap in flux flow in the radial direction; and thus, even though there is an attraction between the stator and armature, this does not result in any increase in force in the axial or operational direction of the force motor. In order to maximize flux flow (minimizing air gaps). this dimension is made as small as possible (minimizing reluctance of the flux flow path), although a sufficient clearance must be maintained to allow for relative movement between the stator and armature.
Another force motor of the prior art is illustrated in FIG. 2. The motor 34 of FIG. 2 utilizes four coils 36, 38, 40, 42 annularly centered on shaft and armature assembly 44, which is axially slidable to the right or left. The electrical energizing of any one coil establishes lines of magnetic flux which is called a "lane", and the energizing of all four coils provides four lanes. Spacers 46, 48 and centering springs 50, 52 help keep the shaft and armature assembly 44 centered in relation to working air gaps 54 and 56 and at a constant distance from the coils 36, 38, 40, 42. Permanent magnets 58, 60 are situated between pole pieces 62, 64 and spacers 46, 48, and have both North poles facing towards each other, thus generating static flux paths 66, 68 (solid lines). When coils 36, 38, 40, 42 are all electrically energized in parallel so that they all help generate flux path 70 (dotted lines), shaft and armature assembly 44 will be shifted to the left because of the cumulative effect of permanent magnet flux path 68 and coil-generated flux path 70 across air gap 54. A reversal of electric polarity in coils 36, 38, 40, 42 causes coil-generated flux path 70 to be oriented in the reverse direction (not shown), thus adding cumulatively to static flux path 66 across air gap 56, causing shaft and armature assembly 44 to be shifted to the right.
A major advantage of the motor of FIG. 2 over that of FIG. 1 is the fact that three levels of redundancy are built into the motor of FIG. 2, while the motor of FIG. 1 has none. If one, two or three of the coils of the motor of FIG. 2 fail, the remaining coil[s] can effectively actuate the shaft and any associated spool valve, if the coils are electrically connected to parallel drivers. The motor of FIG. 1, on the other hand, with only two serially-connected coils cannot provide any extra levels of redundancy.
There are a number of drawbacks to the motor of FIG. 2. First, the magnetic circuits of each coil share the same core structure so that voltage transients caused by a malfunction in one coil can induce undesirable voltages through the other coil[s], causing instability and erratic performance. Second, heat generated by shorted coils may be transferred to adjacent coils causing deteriorating performance and/or additional coil failure. Third if only a single energized coil on one end is energized, asymmetrical flux may be generated through the respective air gaps, resulting in asymmetrical attractive forces acting upon the armature through the respective air gaps, depending upon coil polarity.
Fourth, continued stacking of coils to increase the redundancy safety factor causes the length and weight of the motors to increase prohibitively, especially in aircraft use where space and weight are at a premium. Fifth, the motor of FIG. 2 uses a magnetically soft material between the working air gap and the magnet, causing the flux path in the gap to be less defined.
Therefore, there exists a need in the art for a multilane force motor possessing several layers of redundancy which provides symmetrical moving forces upon the moving parts during multilane failure, and which electrically and magnetically isolates all lanes in case of a coil short circuit or open circuit.